generate warning for reexport name conflict

[hardfist]

when I reexport multi module

• a.js

export const common = 42;
export const answer = 12;

• b.js

export const secret = 23;
export const common = 42;

• index.js

export * from './a';
export * from './b';
console.log('index');

run

$ esbuild index.js --bundle --format=esm

generate result

// answer.js
var answer = 12;

// secret.js
var secret = 23;

// index.js
console.log("xxx");
export {
  answer,
  secret
};

and the common is lost because it conflicts in a.js and b.js and not generates warning, which is dangerous

[evanw]

Makes sense. I don't think this should be a warning by default because this is a normal part of how JavaScript works. I disagree that it's always dangerous. You may have use cases where it doesn't matter for example, and you may not be able to do anything about the warning if you don't control the source code of the files you are re-exporting from. But I can add a way to opt-in to logging this with something like --log-override:ambiguous-import=warning.

[hardfist]

I don't know it's a standard behavior, but it seems that when these files are compiled into commonjs, the runtime behaves differs from the esm result.